Recently, with rapid popularization of the Internet and a rapid increase of connections between LANs within industry, not only the number of outgoing calls for communication is increasing, but also the capacity of transmitted contents data is increasing as in motion pictures, thereby causing a problem in a rapid increase in data traffic. Therefore, the WDM (Wavelength Division Multiplex Transmission) system has achieved remarkable development and becomes widespread, in order to prevent the communication performance from decreasing due to an increase in the data traffic.
In the WDM system, a plurality of optical signals are respectively put on different wavelengths, thereby realizing large-capacity transmission 100 times as large as the conventional transmission, only by one fiber. Particularly, the existing WDM system uses an erbium-doped fiber amplifier (hereinafter referred to as EDFA), thereby enabling broadband and long distance transmission. The EDFA is an optical fiber amplifier which employs the principle that when light is made to passes through a special optical fiber doped with an element called erbium by an excitation laser of a wavelength of 1480 nm or a wavelength of 980 nm, the light in the wavelength range of 1550 nm, which is a transmission signal, is amplified in the special fiber.
On the other hand, the EDFA is a centralized optical amplifier in which portions exciting an optical signal are centralized, and has a limitation in that there is a loss of a transmission line optical fiber that leads to accumulation of noises, and non-linearity that causes distortion or noise in signals. Further, the EDFA enables optical amplification in a wavelength range which is determined by the bandgap energy of erbium, and hence it is difficult to make the bandwidth wide in order to realize further multiplexing.
Therefore, as an optical fiber amplifier which replaces the EDFA, a Raman amplifier is in the spotlight. The Raman amplifier is a distribution type optical amplifier which does not require a special fiber such as erbium-doped fiber as in the EDFA, and uses a normal transmission line fiber as a gain medium. Hence, the Raman amplifier has a characteristic in that it can realize a transmission band which is a broad band and has a flat gain, as compared with the conventional WDM transmission system using the EDFA as the base.
FIG. 9 is a block diagram which shows the schematic construction of a conventional Raman amplifier. In FIG. 9, the Raman amplifier comprises an optical coupler 120 provided on a transmission line 99, optical isolators 111 to 113, and a high-power pumping unit (HPU) 130.
FIG. 10 is a diagram which shows a configuration example of the HPU 130. In FIG. 10, the HPU 130 comprises six laser units LD1 to LD6 having different oscillation center wavelengths, and a Mach-Zehnder type WDM coupler 131. Respective laser units LD1 to LD6 have two Fabry-Perot semiconductor laser modules 134 having same oscillation center wavelengths, so that the laser outputs from respective semiconductor laser modules 134 are wavelength-stabilized by a fiber Bragg grating (FBG) 133, and coupled by a polarization beam combiner (PBC) 132 to thereby form one output.
The polarization multiplex by this PBC 132 is a measure which increases the output power of each oscillation center wavelength and reducing the polarization dependency of the Raman gain. Specifically, by using the polarization multiplex, a laser beam in the TE mode and a laser beam in the TM mode, which have the same wavelength, can be mulchplexed without any interference, thereby enabling a reduction in an optical loss.
In this manner, the HPU 130 needs to perform amplification over the multiplexed signal lights having a plurality of wavelengths (channels), and hence is composed of a plurality of laser units having different oscillation center wavelengths. The laser outputs from the respective laser units LD1 to LD6 are coupled by a WDM coupler 131 and output as a high-output multiplexed exciting light. The exciting light output from the HPU 130 passes through the optical fiber, which is the transmission line 99, through the optical coupler 120. In FIG. 9, there is shown an example of rear side excitation, wherein the exciting light coupled by the optical coupler 120 passes through the inside of the transmission line 99 towards the direction opposite to that of the signal light.
Since the high-output exciting light passes through the inside of the transmission line 99, based on the material characteristic of the optical fiber as a transmission media, the Raman scattered light occurs in the wavelength region shifted to the longer wavelength side than the exciting light by about 110 nm, and the energy of the exciting light is changed to a signal light through an induced Raman scattering process. As a result, the signal light is amplified.
As described above, the Raman amplifier is an amplifier which can use an existing optical fiber as an amplification medium, to directly amplify the signal light. It is different from the EDFA in terms of the amplification medium, the number of used excitation light sources, and the excitation power. In the EDFA, for a light source which excites the erbium-doped fiber, one having the same construction as that of the HPU 130 can be used.
For the Raman amplifier, in addition to the rear side excitation method in which a signal light is excited from the rear side, as in the Raman amplifier shown in FIG. 9, there are a front side excitation method in which the signal light is excited from the front side, and a bi-directional excitation method in which the signal light is excited bi-directionally. The one mainly used as the Raman amplifier at present is the rear side excitation method shown in FIG. 10. The reason is that the front side excitation method in which a weak signal light progresses in the same direction together with the strong exciting light has a problem in that the excited optical power fluctuates. Therefore, it is desired to develop as table excitation light source also applicable to the front side excitation method. That is to say, if a semiconductor laser module using the conventional fiber grating is used, there is a problem in that the applicable excitation method is limited.
The Raman amplification in the Raman amplifier is based on a condition that a polarization direction of the signal light coincides with a polarization direction of the exciting light. That is to say, the Raman amplification has a polarization dependency of the amplified gain, and it is necessary to reduce the influence caused by a deviation between the polarization direction of the signal light and the polarization direction of the exciting light. In the instance of the rear side excitation method, the signal light has no problem since the polarization becomes random during propagation. In the instance of the front side excitation method, however, the polarization dependency is strong, and it is necessary to reduce the polarization dependency by means of cross polarization multiplex, depolarization or the like of the exciting light. That is, it is necessary to reduce the degree of polarization (DOP).
Further, with the Raman amplification, the obtained optical gain is relatively low. Hence, a high output excitation light source for Raman amplification has been desired.